What's the explanation for the 245 being faster? I run 255 and it never crossed my mind that going to a 245 would be faster.
Basically this below:
Better gearing, significantly lighter both as sprung and rotational weight, and sidewalls better supported within the rules of STH.
It truly feels like a different car when you compare 255 40 17 and 245 40 17.
And if your on 660's, it's even worse. They're closer to 265 in the 255 size. So damn heavy.
I cycled between (6) V730s all last year. (4) 245/40R17s I ran initially, and then because I started doing a bunch of autocrossing I picked up a pair (2) of 255/40R17s. For most of the year I ran the 245 square setup on track.
I ran the 245 square setup at the first autocross of the year:
https://www.datadrivenmqb.com/tripreports/tsccax1
Tried the 255s at the next event:
https://www.datadrivenmqb.com/tripreports/tsccax2
Ran the 245s square in May, set my personal fastest lap ever at VIR:
https://www.datadrivenmqb.com/tripreports/vir-with-tscc-may-2023
The next autocross event I ran the 255s again with a codriver... I think their main benefit is heat resistance... but enough to matter, I'm not sure:
https://www.datadrivenmqb.com/tripreports/tsccax3
Had some oil ingestion problems on track later that summer. Kept autocrossing on the 255/245 setup in the meantime, also autocrossed some other peoples' cars as well. They didn't see THAT MUCH use.
Tried out RCO ball joints at stock ride height at Fastivus in September (I really just wanted the camber though - it was a quick experiment in an attempt to add camber) at Fastivus on the 255/245. Nothing to really learn there tire wise. The ball joints were awful. Removed them immediately afterwards. Not sure if it was the added bump steer or the messed with roll center height. Will find out this year since I'm throwing APR springs on to drop it a bit.
Oktoberfast at VIR: This was the first chance to actually push on track with the 255/245s at VIR (which I'm very familiar with at this point). The grip felt down, primarily in the ability to trail-brake effectively. I posted a few graphs here comparing the "friction circles" from May (245 square) vs October (255/245):
https://www.datadrivenmqb.com/tripreports/oktoberfast23
December at VIR: Drove on the 255/245 setup again. Didn't go any faster again, and the car never felt confident enough to push 120mph in the esses like back in May.
In a nut shell there are a lot of potential variables... but the car has never even anecdotally "felt" as good as it was on the 245s square. This COULD vary from tire to tire, but generally speaking I think a LOT of autocrossers just copy what "the fast guys do" and too much "information" gets passed from one person to another. Without any proper testing being done.
The FAST guys are fast regardless of what they drive. It literally doesn't matter. The other 99% of autocrossers make lemming decisions because XYZ Alien autocrosser does it... and there are VERY few of even the top guys who do
real proper testing. The Aliens can be fast in
spite of what they're running, not
because of it.
Without doing a proper A/B/A test same day, two sets of tires/wheels which have already been "scrubbed in" identically and given a proper rest period first, you just won't know for 100% sure. It's fucking expensive and time consuming, and the human element will also always be at play as well. Then throw in that some people just plain drive cars that are set up differently better than others. And also there's always the likelihood that a tire change MAY benefit from some other suspension tweak that would NOT normally benefit the other. This applies for different brand tires as much as it does for one size of tire to another.
I don't consider what I did as true "testing". It was just comparing some data from a few separate trips.
I WILL say it is incredible how noticeable it is just driving around though. It feels different immediately even just pulling out of the driveway. The steering is so much heavier with 255s and considerably less sharp.
FWIW my Mazda2 best it ever felt was on 205s on 15x8in wheels. It was a noticeable difference going from 15x7.5 to 15x8 wheels on that thing. I ran a set of 225s on 9in wide wheels up front at a few events in that car, and it felt good laterally but the car was just so Goddamn slow to accelerate. There was a ~3mpg difference on average between the 225s on 9s and 205s on 8s on that car.
